The harddrive is still used, I never said it is not used.
What I say is that with ReadyBoost, you only use it when the write buffer is full. In between the drive is not spinning. Its conserving power. Only when the write buffer is reached its limit will the drive kick in and say, time to clean the buffer and does what ever it needs to do to clear it. The net effect is :
1) The HD is not spinning always, it only starts when it is needed.
2) Performance increase, because the files are cached on the go.